Tractive Effort, Horsepower and Slugs

[dharmon]

Okay, I think I'm good on the general horsepower vs. tractive effort concept (starting power vs speed) but now I have a couple of questions:

1. I was on the GATX leasing site. The GP38s are listed as2000 HP, weigh 250K average, and have a TE of 58K/lbs @ 10 MPH. The GP40s show as 3000 HP units, weighing 257K average, but have a lower TE at 55.4K/lbs @ 10 mph. Both show to have the same traction motors. I was under the impression that the greater weight would give a greater TE, but in this case it seems it doesn't. Similiar but less. Why would this be?

2. In Gabe's thread about reduced horsepower in rebuilds, one of the members mentioned that a shortline may not need the additional 1000 HP of a GP40 when a GP38 with essentially the same TE would suffice. Makes sense to me. Now that being said.....St Lawrence and Atlantic uses GP40s as mothers for their RM1 RoadMATES. Many of the pictures I have seen show two mother / slug units in consist (4 units total) in the lead. My understanding is that about 25 mph after the train gets rolling, the slugs get shut down......extra tractive effort to start the train, then HP from the 2 GP40 mothers take over once its going.....so I guess the question would be why GP40s instead of GP38s as mothers? Is the extra 1000 HP what is needed to pull the train once in motion or is it also needed to generate the extra juice for the the slug or both?

Dan

[Anonymous]

I will proceed to pronounce my ignorance in all it's entirety on the problem of Tractive Effort.

I understand T/E as a value of work that is placed on the rails to pull against a mass (train) by the wheels. In steam engines smaller diameter wheels were able to maintain high T/E values against larger wheels which could not really put as much on the track.

The WEIGHT on the wheels determines what work the wheels can place on the rails. This is limited by availible traction. The number of wheels that can be on the rails doing the work creates the T/E

Finally.. Both the GP38 and GP40 are 4 axle units. The Gross Weights on them are very similar. The extra 7,000 pounds of weight will not significantly add to the T/E


If I was on grades and hills I go with the GP38 If I was on the level I'll choose the GP40 as I may be able to do the same work of 0 to 10 mph in "less" time perhaps haul a bigger train.

I think the GP40 will be too slippery. I would want a engine that can get down and pull, not carry around excess horsepower that would be wasted due to lack of gross weight etc.

I would want 6 axle units with more gross weight to create a T/E of perhaps 70,000 pounds or more. The addition of the extra set of axles may save me from having to buy a second desiel and give me a better load on the bridges etc.

A slug is a desiel with no prime mover, just the traction motors with perhaps more weight added, it would need a mother unit to feed it.

[martin.knoepfel]

another question as to slugs.

wouldn't it make sense to equip them with an extra-large fuel-tank, to avoid refuelling-stops? the larger tank can occupy the space under the empty hood. of course, you need a flexible pipe-connection between mother and slug.

[ericsp]

Do not confuse work and force. Work is the energy required to move an object using a force over a distance. Tractive effort is a force. Actually, the minimum speeds are different, 10.7 mph for all types of GP38s and 11.3 mph for all types of the GP40s. I do not know for sure why the TE is lower for the GP40s than for the GP38s, however, I can take a few guesses. One reason my be the if the locomotive is pulling its maximum load (force), the small difference in speed causes the wheels to slip. I do not know if there are locomotive wheels made from different material, if there are, it might be that since the GP38 is use more for low speed, high force service and the GP40 primarily used for higher speed, lower force service that the GP40 has wheels made from a material with a lower coefficient of friction. Maybe they were inconsistant in their test and have the coefficient of static friction for the GP38 and the coefficient of kenetic friction for the GP40.

Probably the reason why slugs do not have extra large fuel tanks is that diesel fuel has a lower density than steel or concrete.

[daveklepper]

Different gear ratios would explain the difference. The higher tractive effort results from a gear ratio that has the wheels turning slower for given speed of rotation of the motors.

[Randy Stahl]

QUOTE: Originally posted by martin.knoepfel

another question as to slugs.

wouldn't it make sense to equip them with an extra-large fuel-tank, to avoid refuelling-stops? the larger tank can occupy the space under the empty hood. of course, you need a flexible pipe-connection between mother and slug.

The flexible coupling between units is only one reason not to do this, of course when the fuel is emptyed from the slug fuel tank , there goes your adhesion. Don,t forget that you will also need another fuel pump.
Randy

[Randy Stahl]

QUOTE: Originally posted by daveklepper

Different gear ratios would explain the difference. The higher tractive effort results from a gear ratio that has the wheels turning slower for given speed of rotation of the motors.

Probably not, 62:15 gears are pretty much standard for all EMD freight engines.
Randy

[dharmon]

QUOTE: Originally posted by HighIron2003ar



A slug is a desiel with no prime mover, just the traction motors with perhaps more weight added, it would need a mother unit to feed it.

What I was getting at was.. Does the extra 1000 HP of the GP40 make it more advantageous because that HP is going to support the slug's tractive effort (ie powering it) without affecting it's own tractive effort or does that extra 1000 HP over the GP38 let it move the train at say 40 mph vice 20 mph once it gets moving and the slugs shut down? Why would they choose the GP40 as a mother for the RM1s vice a GP38?

[Randy Stahl]

QUOTE: Originally posted by dharmon

QUOTE: Originally posted by HighIron2003ar



A slug is a desiel with no prime mover, just the traction motors with perhaps more weight added, it would need a mother unit to feed it.

What I was getting at was.. Does the extra 1000 HP of the GP40 make it more advantageous because that HP is going to support the slug's tractive effort (ie powering it) without affecting it's own tractive effort or does that extra 1000 HP over the GP38 let it move the train at say 40 mph vice 20 mph once it gets moving and the slugs shut down? Why would they choose the GP40 as a mother for the RM1s vice a GP38?

With a GP-38 it may not get up to 20 mph very fast if at all. Remember that you must divide the current output from the main generator by 8 instead of 4. The AR-10 is a constant kilowatt machine. After 20 mph the slug really isn't needed anymore . Time to go into parellell and really get going.
The GP -38 is not a low speed locomotive at all. On the Milw we used them for short fast trains.
As far as I know, for a while now railroad wheels are all made from the same thing, steel.
Randy

[Anonymous]

dharmon, I really cannot tell you the answer to that question. Randy Stahl thanks for answering it.

[dharmon]

Thanks fellas....'ppreciate it.

Dan

[ericsp]

QUOTE: Originally posted by daveklepper

Different gear ratios would explain the difference. The higher tractive effort results from a gear ratio that has the wheels turning slower for given speed of rotation of the motors.

They listed all as 62:15.

[ericsp]

QUOTE: Originally posted by Randy Stahl

QUOTE: Originally posted by dharmon

QUOTE: Originally posted by HighIron2003ar



A slug is a desiel with no prime mover, just the traction motors with perhaps more weight added, it would need a mother unit to feed it.

What I was getting at was.. Does the extra 1000 HP of the GP40 make it more advantageous because that HP is going to support the slug's tractive effort (ie powering it) without affecting it's own tractive effort or does that extra 1000 HP over the GP38 let it move the train at say 40 mph vice 20 mph once it gets moving and the slugs shut down? Why would they choose the GP40 as a mother for the RM1s vice a GP38?

With a GP-38 it may not get up to 20 mph very fast if at all. Remember that you must divide the current output from the main generator by 8 instead of 4. The AR-10 is a constant kilowatt machine. After 20 mph the slug really isn't needed anymore . Time to go into parellell and really get going.
The GP -38 is not a low speed locomotive at all. On the Milw we used them for short fast trains.
As far as I know, for a while now railroad wheels are all made from the same thing, steel.
Randy

Do they always use the same steel, or have different compositions of steel been used? Do you think that the different tractive efforts is a result of wheel slip since the locomotives are pulling all the weight they can and the GP40 has a higher minimum continuous speed? I calculated coefficients of friction of 0.232 for the GP38 and 0.216 for the GP40, which is only a 3.672 percent difference.

[PNWRMNM]

Ericsp,

The difference in TE is due to the difference in MCS, which is due to higher amperage for at any given throttle position for GP40 than GP38. Both are likely limited by adhesion, which is why the values are so close together. It is possible that GP 38 is adhesion limited and GP 40 is limited by motor current. You would have to look at both power curves side by side to figure it out and I havent got them handy.

Mac

[daveklepper]

I always thought a variety of gear ratios was avaible on all EMD power. When did this end?

[adrianspeeder]

I thought the Geep 40 was turboed and the 38 wasnt, so yards and such want a 38 not to deal with turbo problems.

Adrianspeeder

[dharmon]

QUOTE: Originally posted by adrianspeeder

I thought the Geep 40 was turboed and the 38 wasnt, so yards and such want a 38 not to deal with turbo problems.

Adrianspeeder

I'd agree with that. I was inquiring more with regards to road slugs.

[CSSHEGEWISCH]

Slugs are inherently low-speed motive power, and are often used with a high-horsepower road unit to get the extra tractive effort at low speeds out of the extra horsepower that otherwise could not be used due to adhesion limits. A GP40/slug set would be roughly equivalent to 2 GP7's. Notice that most road slugs are paired with GP40's (CSX and PAL) and SCL's MATE's were paired with U36B's.

CSX road slugs and perhaps others are equipped with fuel tanks to supply extra fuel to the mother unit.

[dharmon]

QUOTE: Originally posted by CSSHEGEWISCH

Slugs are inherently low-speed motive power, and are often used with a high-horsepower road unit to get the extra tractive effort at low speeds out of the extra horsepower that otherwise could not be used due to adhesion limits. A GP40/slug set would be roughly equivalent to 2 GP7's. Notice that most road slugs are paired with GP40's (CSX and PAL) and SCL's MATE's were paired with U36B's.

CSX road slugs and perhaps others are equipped with fuel tanks to supply extra fuel to the mother unit.

That's what I was trying to confirm. I suspected that the reason the GP40s were used as mothers over GP38s because the higher horsepower was available to power the slug initially to get the train moving, then to have sufficient horsepower to maintain speed once on step, which a lower HP unit may not be able to do.

It seems that one of the hybrid locomotives could be used as sort of a self powered slug in a sense, to help get a train moving or at low speeds, and shut down at higher speed. Just a thought.

[Anonymous]

I dont understand why the slugs CANNOT run at speed with the mother units at 50 mph on a train??

In fact why not equipt rolling stock with traction motors and run eletrical feeds in the entire train?! Let the mother units feed the train and perhaps we can get really good numbers in performance versus money costs.

[adrianspeeder]

Any of these high horsepower locos actually cant use all the horsepower at low speeds because it is "too much" for the traction motors to handle. So it is sent off to other traction motors in slugs to let the loco use most of its horsepower and generating capacity. At higher speeds the traction motors can "take" all the horsepower and electrical load so extra TMs arnt needed.

This is my "rough" idea, so fill in if needed.

The cost alone would nix your idea highiron. Imagine doing all that maintance, for something that actually perfoms well now. Also the cables would weigh as much as the train to factor in resistance and the load required.

Adrianspeeder

[Leon Silverman]

To elaborate on Adrianspeeder statement about the horsepower limitations of traction motors, it behoves us to remember that we are dealing with DC traction motors, only.
DC traction motors could not sustain their maximum current capacity below about 11 mph. One of the reasons for the success of the EMD F units was that they would idle down if this was attempted while the Baldwin's would simply hunker down at these low speeds until they burned themselves out.
The slugs were usually constructed from older units with worn out prime movers and were a cheaper alternative to purchasing additional units. Utilizing the slugs avoided the traction motor burnout that would have resulted if the mother engines were forced to operate continuously at these speeds.
The advent of AC traction motors eliminated the need for slugs since these motors were thermally indistructable, permitting the engines to haul a train up a hill at a walking pace, if necessary, with the prime movers operating at full horsepower. This would be done only if the AC engines were not mu'd with a DC motored locomotive.

[timz]

"The GP38s are listed as2000 HP, weigh 250K average, and have a TE of 58K/lbs @ 10 MPH. The GP40s show as 3000 HP units, weighing 257K average, but have a lower TE at 55.4K/lbs @ 10 mph"

A GP40 that's producing its full horsepower would have a TE (adhesion permitting) of 55000 lb at 16-17 mph. But since GP40s are/were often MUed with SD40s, GP38s, GP9s and other engines that do their best pulling around 11 mph, EMD set up the GP40 control system to taper its horsepower downward below 23 mph. The throttle remains in Run 8, but at 11 mph the GP40 and GP38 are pulling about the same.

"I always thought a variety of gear ratios was avaible on all EMD power. When did this end?"

Sure, it was (is?) available. But RRs almost always chose 62:15 on GP/SD38/40s.

[oltmannd]

QUOTE: Originally posted by dharmon

Okay, I think I'm good on the general horsepower vs. tractive effort concept (starting power vs speed) but now I have a couple of questions:

1. I was on the GATX leasing site. The GP38s are listed as2000 HP, weigh 250K average, and have a TE of 58K/lbs @ 10 MPH. The GP40s show as 3000 HP units, weighing 257K average, but have a lower TE at 55.4K/lbs @ 10 mph. Both show to have the same traction motors. I was under the impression that the greater weight would give a greater TE, but in this case it seems it doesn't. Similiar but less. Why would this be?

2. In Gabe's thread about reduced horsepower in rebuilds, one of the members mentioned that a shortline may not need the additional 1000 HP of a GP40 when a GP38 with essentially the same TE would suffice. Makes sense to me. Now that being said.....St Lawrence and Atlantic uses GP40s as mothers for their RM1 RoadMATES. Many of the pictures I have seen show two mother / slug units in consist (4 units total) in the lead. My understanding is that about 25 mph after the train gets rolling, the slugs get shut down......extra tractive effort to start the train, then HP from the 2 GP40 mothers take over once its going.....so I guess the question would be why GP40s instead of GP38s as mothers? Is the extra 1000 HP what is needed to pull the train once in motion or is it also needed to generate the extra juice for the the slug or both?

Dan

Answer to #1

Normally, the TE rating specified is for the thermal limit the traction motors, so, regardless of weight, you'd have the TE rating at minimum continuous speed.

For an EMD four axle, with 62:15 gears and D77 motors, that would be 55,000#.

To find the minimum continuous speed (MCS), use the formula:

TE (lbf) x speed (mph)/308 = HP

For 2000 HP you get 11.2 mph

For 3000 HP, you get 16.8 mph

But, if you want to run a GP38 with a GP40 and get the max TE out of the consist, you have to "throttle back" the GP40 at speeds below 16.8 mph. EMD offers electronics that does this for you automatically such that the MCS for a GP40 would be about 11 mph.

The weight of the loco will determine what adhesion is needed to get that force to the rail. For a 250,000# GP38 to get 55,000# to the rail, it would require 22% adhesion. This is achievable under good conditions, but the "all weather" rating for IDAC/WS10 EMDs, is more like 18%, so you'd only be able to count on 45,000# TE day in and day out.

Answer to question #2:

A GP38 and a slug will move the same tonnage as a GP40 and a slug. You could repower a locomotive with a Ford Power Stroke diesel @ 200 hp or so and still move the same tonnage.

But, how slow do you want to go? 50% more HP = 50% more speed. You want to go up your max grade at 11 mph or 8 mph? The difference might be significant with respect to your operations.

[dharmon]

QUOTE: Originally posted by oltmannd

QUOTE: Originally posted by dharmon

Okay, I think I'm good on the general horsepower vs. tractive effort concept (starting power vs speed) but now I have a couple of questions:

1. I was on the GATX leasing site. The GP38s are listed as2000 HP, weigh 250K average, and have a TE of 58K/lbs @ 10 MPH. The GP40s show as 3000 HP units, weighing 257K average, but have a lower TE at 55.4K/lbs @ 10 mph. Both show to have the same traction motors. I was under the impression that the greater weight would give a greater TE, but in this case it seems it doesn't. Similiar but less. Why would this be?

2. In Gabe's thread about reduced horsepower in rebuilds, one of the members mentioned that a shortline may not need the additional 1000 HP of a GP40 when a GP38 with essentially the same TE would suffice. Makes sense to me. Now that being said.....St Lawrence and Atlantic uses GP40s as mothers for their RM1 RoadMATES. Many of the pictures I have seen show two mother / slug units in consist (4 units total) in the lead. My understanding is that about 25 mph after the train gets rolling, the slugs get shut down......extra tractive effort to start the train, then HP from the 2 GP40 mothers take over once its going.....so I guess the question would be why GP40s instead of GP38s as mothers? Is the extra 1000 HP what is needed to pull the train once in motion or is it also needed to generate the extra juice for the the slug or both?

Dan

Answer to #1

Normally, the TE rating specified is for the thermal limit the traction motors, so, regardless of weight, you'd have the TE rating at minimum continuous speed.

For an EMD four axle, with 62:15 gears and D77 motors, that would be 55,000#.

To find the minimum continuous speed (MCS), use the formula:

TE (lbf) x speed (mph)/308 = HP

For 2000 HP you get 11.2 mph

For 3000 HP, you get 16.8 mph

But, if you want to run a GP38 with a GP40 and get the max TE out of the consist, you have to "throttle back" the GP40 at speeds below 16.8 mph. EMD offers electronics that does this for you automatically such that the MCS for a GP40 would be about 11 mph.

The weight of the loco will determine what adhesion is needed to get that force to the rail. For a 250,000# GP38 to get 55,000# to the rail, it would require 22% adhesion. This is achievable under good conditions, but the "all weather" rating for IDAC/WS10 EMDs, is more like 18%, so you'd only be able to count on 45,000# TE day in and day out.

Answer to question #2:

A GP38 and a slug will move the same tonnage as a GP40 and a slug. You could repower a locomotive with a Ford Power Stroke diesel @ 200 hp or so and still move the same tonnage.

But, how slow do you want to go? 50% more HP = 50% more speed. You want to go up your max grade at 11 mph or 8 mph? The difference might be significant with respect to your operations.

Thanks! This all helps put some things in better perspective.

Dan

[Anonymous]

So if you have a 14000hp electric that only weighs 200,000lbs and can only provide 85000 TE, vs a 3000hp diesel electric that weighs 400,000 and can provide 100,000 TE, the Diesel could move a heavier train, but the electic would go faster and climb a steeper hill? Just to be clear... or am I confused..

[ericsp]

QUOTE: Originally posted by BR1116

So if you have a 14000hp electric that only weighs 200,000lbs and can only provide 85000 TE, vs a 3000hp diesel electric that weighs 400,000 and can provide 100,000 TE, the Diesel could move a heavier train, but the electic would go faster and climb a steeper hill? Just to be clear... or am I confused..

The electric versus diesel electric is a red herring and will confuse people, so throw that out. The tractive effort will determine the weight of the train that the locomotive can haul. The horsepower available will determine how fast the train can accelerate and what its maximum velocity will be. The grade a train can climb is mainly a function of the tractive effort. If the train is going fast enough and the grade is short enough, then extra speed will help it to climb a grade it might otherwise stall on, however I would guess this is rarely a factor.

Compare trains on the Sunset Route and Tehachapi.
Sunset Route: Light, fast intermodal trains with relatively easy grades. SP bought GP40-2s, GP60s, 8-39Bs, and 8-40Bs for this.
Tehachapi: Long, heavy, slow trains with relatively steep grades. SP used mainly SD40Rs, SD45Rs, SD40T-2s, and SD45T-2s on these.

[oltmannd]

QUOTE: Originally posted by BR1116

So if you have a 14000hp electric that only weighs 200,000lbs and can only provide 85000 TE, vs a 3000hp diesel electric that weighs 400,000 and can provide 100,000 TE, the Diesel could move a heavier train, but the electic would go faster and climb a steeper hill? Just to be clear... or am I confused..

Yes.

[timz]

"... the Diesel could move a heavier train, but the electic would go faster and climb a steeper hill?"

If the high-TE diesel can move a heavier train on any given grade, then it can climb a steeper grade with any given train-- right?

[oltmannd]

Yes. It's 20# TE per trailing ton per percent of grade.